1. Field of the Invention
The present invention relates to a recording apparatus such as a laser beam printer.
2. Description of the Prior Art
Referring to FIG. 1, a known laser beam printer of a single-drum type has a chassis 21 which carries a sheet cassette 22 holding a stack of sheets of paper 24. The sheets of paper 24 are fed one-by-one by means of a sheet feeding roller 23. The sheet fed by the feeding roller 23 is wound around a transfer drum 25 and is held on the drum 25 by a gripper which is not shown. A laser unit 27 is adapted to emit a laser beam which is reflected by a scanning mirror 28 so as to scan the surface of a photosensitive drum 26. Reference numeral 29 designates a primary corona charger for uniformly charging the photosensitive drum 26. The laser beam printer further has developing devices 30y, 30m and 30c which are adapted to develop a latent image on the photosensitive drum 26 with developing color toners of yellow, magenta and cyan, respectively, so as to make the latent image visible in respective colors. The thus formed image is transferred to the sheet of paper 24 on the transfer drum 25 by means of a transfer charger 31. After the completion of the image-forming process, the sheet of paper 24 is separated from the transfer drum 25 by means of a separation claw 32. A reference numeral 33 denotes a conveyer belt which conveys the sheet of paper 24 to a fixing device 34 where the image is fixed. The sheet is then discharged to a sheet discharge tray 35. The electrical potential on the surface of the photosensitive drum 26 is detected by a potential sensor 36. The laser unit 27 is composed of a semiconductor laser, a polygonal mirror, an image-forming lens, and other parts (not shown).
The operation of this known laser beam printer, which has been described with reference to FIG. 1, is as follows.
The photosensitive drum 26 is uniformly charged by means of the primary corona charger 29. A laser beam emitted from the laser unit 27 is reflected by the scanning mirror 28 and is applied to the surface of the photosensitive drum 26 so as to scan the latter in a main scanning direction which coincides with the axial direction of the drum, whereby a latent image corresponding to an image signal is formed. Then, the latent image is developed to make the yellow portions of the image visible by the developing device 30y, using yellow toner. Meanwhile, the sheet of paper 24 has been fed from the sheet cassette 22 and wound around the transfer drum 25. The yellow toner image is transferred from the photosensitive drum 26 to the transfer drum 25 by a corona discharge from the transfer charger 31.
Then, the formation of a latent image and its development are then repeated for the magenta and cyan colors, and the thus formed images using magenta and cyan toners are successively transferred to the sheet of paper 24. After the completion of the transfer of the toner images of the three different colors, the sheet of paper 24 is separated from the transfer drum 25 and is conveyed by the conveyer belt 33 to the fixing device 34, where the toner images are fixed on the sheet of paper 24 by heat and pressure. The sheet of paper carrying the fixed image is then discharged to the discharge tray 35. The photosensitive drum 26 and the transfer drum 25 rotate at the same peripheral speed in the directions of the arrows shown in FIG. 1.
During this operation, the quantity of the electric charge on the photosensitive drum 26 is detected by the potential sensor 36 which delivers to a control section (not shown) a signal for controlling the setting of developing bias, thereby controlling the electric potential of the surface of the photosensitive drum 26.
FIG. 2 shows, by way of example, a 4-drum type laser beam printer. In this Figure, the same reference numerals are used to denote the same parts as those in FIG. 1.
This printer has a first laser unit 41.sub.C which is adapted for forming a latent image on a photosensitive drum 45.sub.C for cyan color, in accordance with color picture data contained in an image signal. The printer also has a second laser unit 41.sub.M which is adapted to form a latent image on a photosensitive drum 45.sub.M for magenta color, in accordance with the color picture signal. Similarly, a third laser unit 41.sub.Y forms a latent image on a photosensitive drum 45.sub.Y for yellow color, in accordance with the color picture data, and a fourth laser unit 41.sub.BK forms a latent image on a photosensitive drum 45.sub.BK for black color, in accordance with the color picture data.
Toner hoppers 42.sub.C, 42.sub.M, 42.sub.Y and 42.sub.BK are charged with toners of respective colors: namely, cyan, magenta, yellow and black. Transfer charges 44.sub.C, 44.sub.M, 44.sub.Y and 44.sub.BK, which are associated with respective photosensitive drums, operate to transfer the toner images to the sheet of paper 24 which is being conveyed by a conveyer belt 43. Each of the laser units 41.sub.C, 41.sub.M, 41.sub.Y and 41.sub.BK has a semiconductor polygonal mirror which is adapted to direct the laser beam to the associated photosensitive drum thereby scanning the drum in the direction of the axis of the drum.
In operation, the laser units 41.sub.C, 41.sub.M, 41.sub.Y and 41.sub.BK emit laser beams which are turned on and off in accordance with the color picture data. The laser beams are applied to respective photosensitive drums 45.sub.C, 45.sub.M, 45.sub.Y and 45.sub.BK which have been uniformly charged beforehand, so that latent images of respective colors are formed on the photosensitive drums. These latent images are changed into visible images by the toners of cyan, magenta, yellow and black colors supplied from respective toner hoppers 42.sub.C, 42.sub.M, 42.sub.Y and 42.sub.BK. The visible toner images of respective colors are then transferred by the transfer chargers 44.sub.C, 44.sub.M, 44.sub.Y and 44.sub.BK onto the sheet of recording paper 24 which is being conveyed by the conveyer belt 43. The transferred images are then fixed by the fixing device 34 and the sheet of recording paper 24 carrying the fixed image is discharged to a discharge tray 35.
The laser beam printer of single-drum type shown in FIG. 1 suffers from the problem that the transfer drum 25 must have a large circumferential length corresponding to the length of the sheet of recording paper to be processed. Thus, when the laser beam printer is required to process a recording paper sheet of, for example, A-3 size, the transfer drum 25 is required to have a considerably large diameter with the result that the overall size of the laser beam printer is increased to an impractical extent.
On the other hand, the laser beam printer of the 4-drum type shown in FIG. 2 encounters a problem due to the different rates of degradation in the sensitivity occurring in the photosensitive drums 45.sub.C, 45.sub.M, 45.sub.Y and 45.sub.BK with use. In other words, different degradation rates cause relative differences in the sensitivity of these photosensitive drums. If one or more of the drums are renewed, the balance of the colors in the reproduced image is impaired with the result that the quality of the print image deteriorates to such an extent that, for practical purposes, the acceptable reproduction of the color image is impossible to achieve.
Another problem encountered with the 4-drum type laser beam printer shown in FIG. 2 is that, if the level of the laser power exceeds a predetermined control limit in one or more of the laser units 41.sub.C, 41.sub.M, 41.sub.Y and 41.sub.BK, the color balance is changed resulting in deterioration of the hues of the reproduced color image.